Thermoformable acoustic sheet

ABSTRACT

A thermoformable acoustic sheet formed by a compressed fibrous web includes high melt fibres and adhesive thermoplastic fibres in which the adhesive fibres are at least partially melted so that in the compressed web the adhesive fibres at least partially coat the high melt fibres and reduce the interstitial space in the fibre matrix. Also included are methods of producing a thermoformable acoustic sheet which includes heating a fibre web including high melt and adhesive thermoplastic fibres to at least partially melt the adhesive fibres and compressing the web to form a sheet so that the adhesive fibres at least partially coat the high melt fibres to reduce the interstitial space in the fibre matrix.

BACKGROUND OF THE INVENTION

This invention relates to materials for acoustic absorption. Moreparticularly it relates to thermoformable acoustic sheets.

Sounds absorption is required in a wide variety of industrial anddomestic applications. In many of these applications it is desirablethat the acoustic material conforms to the shape of a surface forexample or otherwise retains a particular shape. In such applications itis desirable that the acoustic sheet can be heat moulded to the requiredshape to provide relative ease and speed of production. Sound absorptioncan be a function of depth of air space, air flow resistance, mass,stiffness and the acoustic impedance of any porous media behind theacoustic sheet. Therefore, adding a third dimension for example bymoulding to a required shape increases stiffness and can add practicaland aesthetic value. Importantly a three dimensionally shaped materialprovides its own air space. The shape therefore has a major influence onsound absorption and stiffness. One particular application for heatmouldable or thermoformable acoustic sheets is in the automotiveindustry, in particular, in under bonnet insulators for motor vehicles.Existing under bonnet insulators use moulded flibreglass insulators forsound absorption. In these products resinated fibreglass, or felt issandwiched between two layers of non-woven tissue and subsequently heatmolded to form a so called “biscuit” with sealed edges. The difficultiesassociated with this product include the fact that the moulding processis relatively slow taking up to 2½ minutes per moulded part.Additionally, the use of resinated fibreglass is undesirable because ofis inherent undesirable handling problems while the resins can releasetoxic gases during the moulding process.

Other examples of applications for thermoformable sheets in theautomotive industry include wheel arch linings, head linings and bootlinings.

Attempts to produce a suitable thermoformable material fromthermoplastic textile for underbonnet insulator have been unsuccessfuldue to one or more of the failure of the materials to meet requirementsof low sag modulus typically encountered at operating temperatures,unsuitable moulding performance, and lack of uniformity of air flowresistance required for acoustic absorption performance.

It is an object of this invention to provide a thermoformable acousticsheet and a method of producing such a sheet that will at least providea useful alternative.

SUMMARY OF THE INVENTION

In one aspect this invention provides a thermoformable acoustic sheetformed by a compressed fibrous web including high melt and adhesivethermoplastic fibres. During forming the adhesive fibres are at leastpartially melted so that in the compressed web the adhesive fibres atleast partially coat the high melt fibres and reduce the interstitialspace in the fibre matrix.

In one form of the invention, the thermoplastic fibres are treated withan adhesive coating to increase the airflow resistance.

In another form of the invention, the thermoplastic fibres are treatedwith a coating formed by one or more further webs of thermoplasticfibres to increase the air flow resistance.

Preferably the further web contains a substantial amount of adhesivefibre.

In another aspect this invention provides a method of producing athermoformable acoustic sheet including the steps of heating a fibre webincluding high melt and adhesive thermoplastic fibres to at leastpartially melt the adhesive fibres and compressing the web to form asheet. In the compressed sheet the adhesive fibres at least partiallycoating the high melt fibre to reduce the interstitial space in thefibre matrix.

In one form of the method of the present invention, the sheet is treatedwith an adhesive coating to increase the air flow resistance.

In another form of the method of the present invention, thethermoplastic fibres are treated with a coating formed by one or morefurther webs of thermoplastic fibres to increase the air flowresistance.

The compression of the fibrous material under heat and pressure resultsin the at least partial melting of the adhesive fibre which acts as aheat activatable binder to at least partially coat and join to the highmelting fibre thus reducing interstitial space in the fibre matrix andcreating a labyrinthine structure that forms a tortuous path for airflow through the fibre matrix. The high melting fibre remainssubstantially intact, although some softening is acceptable and can actas a reinforcement in the acoustic sheet.

Preferably, the acoustic sheet has a total air flow resistance ofbetween 275 and 1100 mks Rayl, more preferably 600-1100 mks Rayl andeven more preferably 900-1000 mks Rayl. Such air flow resistance valuesof the acoustic sheet result in effective absorption of sound forapplications such as hood or under bonnet insulation. In this regard theacoustic sheet produced in accordance with the present invention exhibitthe acoustic behaviour of a porous limp sheet. Porous limp sheetsdisplay superior sound absorption at low frequencies.

Preferably, the thermoformable acoustic sheet has a low sag modulus attemperatures up to about 150° C.

The fibrous material can be a combination of fibres of various denier.The high melt fibres are 12 denier or below, 6 denier or below and/or 4denier or below. The adhesive fibres are 8 denier or below, 6 denier orbelow, 4 denier or below and/or at about 2 denier.

The fibrous material can be selected from, but not limited to,polyester, polyethylene terephthalate (PET), polyethylene butylphthalate(PBT), polyethylene 1,4-cyclohexanedimethanol (PCT), polylactic acid(PLA) and/or polypropylene (PP). Fibre with special characteristics suchas high strength or very high mailing point can also be used. Examplesinclude Kevlar™, Nomex™ and Basofil™. Alternatively, the high meltingpoint fibres may be substituted by natural fibre such as wool, hemp,kanet etc.

The web of fibrous material used to produce the acoustic sheet of thisinvention can be produced from a non-woven vertically aligned high loftthermally bonded material formed by the STRUTO™ process under Patent WO99/61693. Suitable low and high melt materials can be used to providethe respective fibres.

The web of fibrous material used to produce the acoustic sheet of thisinvention can also be produced by cross-lapping and thermal bonding. Theweb can also be produced by carding fibres and consolidation by needlepunching. According to another option the web can be produced by othernon-woven textile manufacturing methods such as melt blown, spun bondetc.

Adhesive fibres are also known as low melt, bonding or binding fibres.Various materials can be used for the high melt and adhesive fibres solong as the adhesive fibre can be partially melted without substantiallymelting the high melt fibre. Some softening of the high melt fibre isacceptable. The high melt fibre preferably has a melting point aboveabout 220° C. The adhesive fibre preferably has a melting point between100 and 160° C., more preferably 120-150° C. and even more preferably135-145° C. It will be appreciated that thermoplastic fibres areavailable in mono and bi component form. A bicomponent fibre can beformed of discrete low and high melting point portions. Heating such abicomponent fibre (“adhesive bicomponent fibre”) results in at leastpartial melting of the low melting point portion leaving the highermelting point portion intact. Therefore in the method of the presentinvention, heating a fibre web results in at least partial melting ofthe adhesive fibres and/or the low melting point portion of any adhesivebicomponent fibres present in the web to at least partially coat andjoin to the high melting fibre. The higher melting point fibres and highmelting point portions of any adhesive bicomponent fibre remain intactafter the compaction process.

The web of fibrous material used to produce the acoustic sheetpreferably has a web weight 1000 g/m² or below, more preferably 800 g/m²or below, even more preferably 600 g/m² or below and even furtherpreferably 400 g/m² or below. The web is typically compressed by between15 and 25 times.

The compression step of the method of the present invention can beundertaken in any suitable known manner, for example in any flat bedlaminator or calendar.

In one embodiment the fibrous material is produced as a single layerwith a high proportion, preferably greater than 50% of adhesive and/oradhesive bicomponent fibre. This may be compacted in a Meyer™ flat bedlaminator at 180-220° C., preferably at 190-200° C., for a period of 1-3minutes, preferably 1.5-2 minutes. The processing conditions can bevaried to alter the thickness and/or other characteristics and thesubsequent air flow resistance of the acoustic sheet.

In one form of the invention, the thermoplastic fibres are treated withan adhesive coating. The coating treatment can be effected in anysuitable known manner, for example by the application of an adhesivefilm or an adhesive powder and subsequent heating. The amount ofadhesive treatment can be adjusted to control the total air flowresistance of the thermoformable acoustic sheet. The adhesive can be acrosslinking adhesive powder. The application rate of powder isdependent on particle size, melting point, melt flow properties andpolymer type. These types of adhesive have an initial curing temperaturethat can be exceeded after curing and cooling without remelting of theadhesive Suitable adhesives include the product SURLYN™ manufactured byDuPont. Typical polymers for the adhesive film and/or powder areco-polyester, polyethylene and/or polypropylene.

In one form of the invention, where the adhesive coating is an adhesivepowder, a layer of non-woven fabric or other material may be laminatedto the compressed thermoplastic sheet using the adhesive powder.

Preferably the compression and coating treatment steps are performed ina single process. That is, heating required prior to the compression andfor adhesive melting (to form the coating) can be a single step beforecompression.

In another form of the invention, the compression of the thermoplasticfibre and the lamination to the non-woven fabric are achieved in asingle process. Preferably a compression and adhesive meltingtemperature of about 200° C. is used.

In another form of the invention, the coating by use of a web ofthermoplastic fibres may be effected by the application of multiple websof fibrous material which are introduced in parallel into the compactionprocess, and compacted concurrently. Alternatively, the web(s) can beintroduced in one or more further compacting steps after the first webof fibrous material including adhesive and high melt thermoplasticfibres has been compacted. The further web(s) of fibrous material caninclude adhesive fibre, adhesive bicomponent fibre and/or high meltfibre. The amount and type of additional fibrous material can beadjusted to control the total air flow resistance of the thermoformableacoustic sheet.

In one form of the invention the thermoformable acoustic sheet can beformed from a first web preferably comprising 10-40%, further preferably20% high melting point fibre and a second web of fibrous material,preferably comprising 60-100% further preferably more than 70%, evenfurther preferably 100% adhesive or adhesive melt bicomponent fibre. Thetwo webs can be compacted concurrently and adhere to each other withoutthe need for an adhesive layer.

In another form of the invention the thermoformable acoustic sheet maybe formed from two webs in which one of the webs may have a relativelylow proportion of adhesive or adhesive bicomponent fibre, such as 10-60%preferably 20-25%. The webs can be compacted as described above.However, in this embodiment, a thermoplastic adhesive layer may berequired to be introduced between the two webs, in the form of a powder.The addition rate of the powder is preferably within the range 10 and 80g/m², more preferably 40-60 g/m². If a film is used rather than a powderit must be thin enough to become permeable during the compactionprocess, preferably from 15-25 microns thick. The adhesive may berequired if the compressed webs exhibit recovery after compaction, or ifthey do not compact sufficiently for adequate sound absorption.

The mouldable acoustic sheet according to this invention has been foundto be particularly suitable for use in automotive applications and Inparticular as an under bonnet acoustic liner. The thermoformableacoustic sheet can be readily formed using a moulding temperature ofbetween 150° and 180° C. and may require use of flame retardant fibresor an additional flame retardant treatment. Suitable additives as flameretardants are deca-bromodiphenyloxide as supplied by Great LakeChemicals. High melt fibres having improved inherent flame retardantcharacteristics may be used, for example a grafted polyester such asTrevira™ CS. The moulded sheet substantially retains the air flowresistance of the unmoulded sheet and thus its acoustic properties.Moreover, the sheet has a low sag modulus at temperatures up to about150° C. and is suitable for use as an under bonnet insulator or liner.

For hood insulator applications, the appearance must be consistent andlow gloss. Appearance can be influenced by the fibre properties andbinder fibres tend to develop gloss during compaction and subsequentmolding. To minimise gloss, the option of using an additional layer offibrous material as the coating with each layer having significantlydifferent fibre blend ratios is preferred. A face web should have arelatively low proportion of binder fibre, preferably 10-20% and a backweb should have a very high binder ration, from 60-100%, preferably 80%.The back web will significantly contribute to flow resistance to assureexcellent sound absorption, whilst the facing web assists in resistingmarring during the process.

The thermoformable material of this invention is also suitable for usein wheel arch linings, head linings and boot linings. In mostapplications the selected air flow resistance of the moulded sheet canbe used in combination with an acoustic cavity or space behind the sheetto achieve desired acoustic absorption.

In another form of the invention the uniform air flow resistance can beat least partially achieved by laminating a textile layer with selectedair flow resistance to the compressed sheet. The layer can for examplebe a slit or perforated thermoplastic film or textile layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to the accompanying drawings and examples, in which:

FIG. 1 is a schematic diagram of a flat bed laminating machine;

FIG. 2 is a plot of normal incidence sound absorption coefficientagainst frequency for tested samples of this invention;

FIG. 3 is a plot of flow resistance versus fibre formulation for sampleshaving a high melt/adhesive fibre ratio of 1:1 and web weight of 600g/m²;

FIG. 4 is a plot of flow resistance versus powder additive weight forsamples having a high melt (6 denier)/adhesive (4 denier) fibre ratio of1:1, and a web weight of 600 g/m²;

FIG. 5 is a plot of sound absorption versus flow resistance for a rangeof samples with a web weight of 600 g/m² at a frequency of 1000 Hz and a60 mm air gap; and

FIG. 6 is a plot of sound absorption versus product weight for a rangeof samples with an air flow resistance of 600 mks Rayls at a frequencyof 500 Hz and an air gap 50 mm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can be implemented using a known laminatingmachine such as a Meyer laminating machine schematically illustrated inFIG. 1. As shown in the drawing the laminating machine 1 includes a websupply roll 2.

The web 3 is fed to a heat contact system 9, which is readily known tothose in the art as including heaters 10 positioned on either side oftwo opposed parallel belts 11 and 12. The belts 11, 12 are thus heatedand in turn heat the web 3 to about 200°. A pair of adjustable pressurerollers 13, 14 bear against the respective belts 11, 12 to compress theweb 3. A subsequent cooling system 15 is provided to cool the compressedproduct.

In the case of a product made using a thermoplastic adhesive powder, theweb 3 is fed from the supply roller through a scatter head 4 whichapplies the thermoplastic adhesive powder to the surface of the web 3. Awinding system 5 for thermoplastic adhesive film 6 is also provided inthe machine 1. It will be apparent to those skilled in one art one orother of the scatter head system 4 or unwinding system 5 forthermoplastic adhesive film 6 is to place adhesive in contact with web3. As described above, the web 3 then continues through heat contactsystem 9 where the thermoplastic adhesive powder is melted under theaction of heated belts 11, 12 as the web 3 is simultaneously compressedunder the action of pressure rollers 13, 14. Cooling system 15 cools thefinal product as described above.

Where a further fabric layer or web is to be provided, a supply offabric or web 7 is stored on a roll 8 prior to entry into the heatcontact system 9 so that the fabric web 7 is fed to the heat contactsystem 9 simultaneously with web 3. Where a thermoplastic adhesive hasbeen deposited on web 3 by scatter head system 4 or unwinding system 5,the heated belts 11, 12 heat the fabric 7 and web 3 to melt theadhesive. Pressure rollers 13, 14 bear against the respective belts 11,12 to force fabric 7 into contact with web 3 and the melted adhesive.Again, as described above, the web 3 is compressed and the coolingsystem 15 cools the compressed and laminated product.

Test Results Example 1

A sample was prepared using the above described machine and tested usingan impedance tube with a 50 mm air gap to ASTME E 1050-90. Theproperties of the sample were:

-   -   carrier formulation 30% polypropylene (adhesive fibre) and 70%        polyester (high melt);    -   web material was a needle punched mixture in roll form;    -   carrier web weight 450 g/m²; and    -   polyester non-woven fabric facing web weight 50 g/m² adhered        with a small (15-20 g) of polypropylene powder.

The average air flow resistance of the sample was 300-400 mks Rayls.

FIG. 2 is a plot of average incident sound absorption versus frequencyfor six randomly selected samples prepared according to this example.

Example 2

A sample was prepared and tested in the same manner as in Example 1 withthe following specifications:

-   -   50% high melt fibre of 6 denier;    -   50% adhesive fibre of 4 denier; and    -   web weight 700 g/m².

The air flow resistance of the sample was in the range of 300-400 mksRayls.

Example 3

A sample was prepared and tested in the same manner as in Example 1 withthe following specifications;

-   -   30% high melt polyester fibre of 6 denier,    -   70% adhesive polyester fibre of 4 denier;    -   web weight 600 g/m².

The air flow resistance of the sample was in the range of 700-850 mksRayls.

Example 4

A sample was prepared and tested in the same manner as in Example 1 withthe following specifications:

-   -   50% high melt polyester fibre of 6 denier;    -   50% adhesive bicomponent polyester fibre of 4 denier; and    -   web weight 600 g/m².

As shown in FIG. 3, the air flow resistance of the sample was in therange of 275-375 mks Rayls.

Example 5

A sample was prepared and tested in the same manner as in Example 1 withthe following specifications:

-   -   50% staple high melt polyester fibre of 6 denier,    -   50% adhesive bicomponent polyester fibre of 2 denier; and    -   web weight 600 g/m².

As shown in FIG. 3, the air flow resistance of the sample was in therange of 450-600 mks Rayls.

Example 6

A sample was prepared and tested in the same manner as in Example 1 withthe following specifications:

-   -   50% high melt polyester fibre of 3 denier;    -   50% adhesive polyester fibre of 2 denier; and    -   web weight 600 g/m².

As shown in FIG. 3, the air flow resistance of the sample was in therange of 550-750 mks Rayls.

Example 7

A sample was prepared and tested in the same manner as in Example 1 withthe following specifications:

-   -   30% high melt polyester fibre of 4 denier;    -   70% adhesive bicomponent polyester fibre of 2 denier;    -   web weight 250 g/m²;    -   spun bonded non-woven fabric polyester with a web weight of 100        g/m²;    -   polyethylene thermoplastic powder at an application rate of 20        g/m²; and    -   dibromophenyloxide flame retardant additive at an application of        25 g/m².

The air flow resistance of the sample was in the range of 700-900 mksRayls.

Example 8

A sample was prepared and tested in the same manner as in Example 1using two webs of fibrous material with the following specifications:

-   -   180 g/m² 30% bicomponent polyester fibre of 2 denier and 70%        high melt black 4 denier polyester fibre; and    -   300 g/m² 100% 2 denier bicomponent fibre.

The two webs of the above specification were introduced to a Meyerlaminator at the following settings.

-   -   pressure 15 KPa;    -   distance between top and bottom belt 1 mm;    -   first bank of heaters temperature 175° C.; and    -   second bank of heaters temperature 190° C.

This resulted in a flow resistance of 900-1100 mks Rayls.

Example 9

A sample was prepared and tested in the same manner as in Example 8:

Web 1

-   -   85% high melt polyester fibre with 4 denier;    -   15% adhesive bicomponent polyester fibre of 2 denier; and    -   web weight 180 g/m².

Web 2

-   -   a 30% staple high melt polyester fibre of 4 denier;    -   70% adhesive bicomponent polyester-fibre of 2 denier; and    -   web weight 250 g/m².

The airflow resistance of the sample was in the range of 700-900 mksRayls.

Example 10

Samples were prepared and tested in the same manner as in Example 1 withthe following specifications:

-   -   50% high melt polyester fibre of 6 denier;    -   50% adhesive polyester fibre of 4 denier:    -   web weight 600 g/m²; and    -   varying application rates of LDPE adhesive powder.

Eight samples were made, each with the application rate of the adhesivepowder varying from 10 g/m² to 80 g/m² in 10 g/m² intervals. A plot ofthe resulting air flow resistance of each sample is shown in FIG. 4.

Test results for a range of acoustic sheets made in accordance with theinvention are illustrated in FIGS. 5 and 6. In FIG. 5, a range ofsamples with a web weight 600 g/m² were tested at a frequency of 1000 Hzwith a 50 mm air gap between the sample and a solid surface for theirsound absorption coefficient against the air flow resistance. FIG. 6illustrates the sound absorption coefficient against product weight(g/m²) for a range of samples having an air flow resistance of 600 mksRayls. The sound absorption coefficients were measured at a frequency of500 Hz with a 50 mm air gap between the samples and a solid surface.

The air flow resistance is dependent on the ratio of binder mat to highmelt fibre. If a low air flow resistance is required, then a smalleramount of binder is required. For a high air flow resistance, the binderratio is significantly higher.

Air flow resistance can vary with fibre size and geometry. Largerdiameter fibres result in lower air flow resistance through a higherporosity.

The foregoing describes a limited number of embodiments of the inventionand modifications can be made without departing from the scope of theinvention.

1-25. (canceled)
 26. A method of producing a thermoformable acousticsheet including the steps of heating a fibre web including high melt andadhesive thermoplastic fibres to at least partially melt the adhesivefibres and compressing the web to form a sheet so that the adhesivefibres at least partially coat the high melt fibres and reduce theinterstitial space in the fibre matrix.
 27. A method of producing athermoformable acoustic sheet according to claim 26 wherein thethermoplastic fibres are treated with an adhesive coating to increasethe air flow resistance.
 28. A method of producing a thermoformableacoustic sheet according to claim 27 wherein the adhesive coating isapplied as a powder at a rate of from 10 to 80 g/m².
 29. A method ofproducing a thermoformable acoustic sheet according to claim 26 whereinthe thermoplastic fibres are treated with a coating formed by one ormore further webs of thermoplastic fibres.
 30. A method of producing athermoformable acoustic sheet according to claim 27 wherein the fibreweb including high melt and adhesive thermoplastic fibres and thecoating formed by one or more further webs of thermoplastic fibres arecompressed concurrently.
 31. A method of producing a thermoformableacoustic sheet according to claim 26 wherein the sheet has a total airflow resistance between 275 and 1100 mks Rayls.
 32. A method ofproducing a thermoformable acoustic sheet according to claim 31 whereinthe sheet has a total air flow resistance between 600 and 1100 mksRayls.
 33. A method of producing a thermoformable acoustic sheetaccording to claim 32 wherein the sheet has a total air flow resistancebetween 900 and 1000 mks Rayls.
 34. A method of producing athermoformable acoustic sheet according to claim 26 wherein the sheethas a low sag modulus at temperatures up to 150° C.
 35. A method ofproducing a thermoformable acoustic sheet according to claim 26, whereinthe high melt fibre has a melting point above about 220° C.
 36. A methodof producing a thermoformable acoustic sheet according to claim 26,wherein the adhesive fibre has a melting point between 100 and 160° C.37. A method of producing a thermoformable acoustic sheet according toclaim 36 wherein the adhesive fibre has a melting point between 120 and150° C.
 38. A method of producing a thermoformable acoustic sheetaccording to claim 37 wherein the adhesive fibre has a melting pointbetween 135 and 145° C.
 39. A method of producing a thermoformableacoustic sheet according to claim 2 wherein the high melt fibres areabout 6 denier or below.
 40. A method of producing a thermoformableacoustic sheet according to claim 39 wherein the high melt fibres areabout 4 denier or below.
 41. A method of producing a thermoformableacoustic sheet according to claim 26 wherein the adhesive fibres areabout 6 denier or below.
 42. A method of producing a thermoformableacoustic sheet according to claim 41 wherein the adhesive fibres areabout 4 denier or below.
 43. A method of producing a thermoformableacoustic sheet according to claim 42 wherein the adhesive fibres areabout 2 denier.
 44. A method of producing a thermoformable acousticsheet according to claim 26 wherein the web of thermoplastic fibres isproduced from non-woven vertically aligned high loft thermally bondedfibres.
 45. A method of producing a thermoformable acoustic sheetaccording to claim 26 wherein the thermoplastic fibres are selected frompolyethylene terephthalate (PET), polyethylene butylphthalate (PBT),polyethylene 1,4-cyclohexanedimethanol (PCT), polylactic acid (PLA)and/or polypropylene (PP).
 46. A method of producing a thermoformableacoustic sheet according to claim 26 wherein the web of thermoplasticfibres has a web weight of about 1000 g/m² or below.
 47. A method ofproducing a thermoformable acoustic sheet according to claim 46 whereinthe web of thermoplastic fibres has a web weight of about 800 g/m² orbelow.
 48. A method of producing a thermoformable acoustic sheetaccording to claim 47 wherein the web of thermoplastic fibres has a webweight of about 600 g/m² or below.
 49. A method of producing athermoformable acoustic sheet according to claim 48 wherein the web ofthermoplastic fibres has a web weight of about 400 g/m² or below.
 50. Amethod of producing a thermoformable acoustic sheet according to claim26 wherein the web of thermoplastic fibres have 50% or more of adhesivefibres or adhesive component fibres and is compressed at a temperaturebetween 180-220° C. for a period between 1 and 3 minutes. 51-52.(canceled)